1. Field
The embodiment discussed herein is related to a phase synchronization apparatus and a digital coherent light receiver.
2. Description of the Related Art
In recent years, attention has been given to a digital coherent light reception method achieved by combining a coherent light reception method and a digital signal processing technique as a next-generation optical communication technique. Techniques relating to the digital coherent light reception method have been disclosed in JP-A-63-187366 and Dany-Sebastien Ly-Gagnon, Satoshi Tsukamoto, Kazuhiro Katoh, and Kazuro Kikuchi, Member, IEEE, Member, OSA, “Coherent Detection of Optical Quadrature Phase-Shift Keying Signals With Carrier Phase Estimation” (JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 24, No. 1, JANUARY 2006, pp. 12-21).
Description is now made of the structure of a digital coherent light receiver according to the related art with reference to FIG. 9. FIG. 9 is a diagram illustrating an example of the structure of a digital coherent light receiver in the related art.
As illustrated in FIG. 9, the digital coherent light receiver has, for example, a 90-degree phase hybrid circuit, an internal oscillator, an 0/E (Optical/Electrical), an ADC (Analog Digital Converter), and a digital signal processing section. The digital signal processing section has, for example, a waveform distortion compensation section, a phase synchronization section, and an identification section.
When signal light is input to such a digital coherent light receiver, the 90-degree phase hybrid circuit mixes the signal light with reference light input thereto from the internal oscillator and converts the mixed light into a signal from which the intensity and the phase information of the signal light can be extracted. Next, the 0/E converts the light signal provided in the conversion by the 90-degree phase hybrid circuit into an electric signal. Then, the ADC converts the electric signal provided in the conversion by the 0/E into a digital signal.
The digital signal processing section processes and demodulates the digital signal while extracting the intensity and the phase information of the light signal from the digital signal provided in the conversion by the ADC. Specifically, the wave distortion compensation section compensates for waveform distortion due to wavelength dispersion in optical fibers, polarization-mode dispersion, polarization fluctuations and the like. Then, the phase synchronization section synchronizes the light signal and local light in terms of the optical frequency and the optical phase and restores the intensity and the phase information of the light signal. The identification section demodulates the signal in accordance with the modulation method based on the intensity and the phase information restored by the phase synchronization section.
The modulation methods which can be implemented in a structure similar to that of the receiver include not only a binary modulation method represented by intensity modulation but also MPSK (Multi-ary Phase Shift Keying) such as QPSK (Quadrature Phase Shift Keying) and QAM (Quadrature Amplitude Modulation), for example.
Next, the structure of the phase synchronization section will be described with reference to FIG. 10. FIG. 10 is a diagram illustrating an example of the structure of a phase synchronizer according to the related art. Among the abovementioned modulation methods, the use of the QPSK modulation will be described.
As illustrated in FIG. 10, the phase synchronization section has a biquadratic operation section, an averaging section, a ¼ division section, an argument calculation section, a delay addition section, and a subtraction section. Out of a signal component and an error component contained in input data, the phase synchronization section calculates the error component and outputs a signal which is provided by removing the error component.
Specifically, the biquadratic operation section extracts the error component from the signal light. The averaging section sums up successive several data and removes the error component extracted by the biquadratic operation section from the data. Then, the ¼ division section returns the error component quadrupled by the biquadratic operation section to the original error component.
The argument calculation performs conversion from complex number into angle. The delay addition section adjusts the delay between the path in an upper portion of FIG. 10 for calculating the argument of the input signal and the path in a lower portion of FIG. 10 for calculating phase noise. Next, the subtraction section subtracts the error component from the input data and outputs the result.
Since there is a trade-off between the number of averaging in the averaging section and variations in SN (Signal to Noise) and the phase noise of the signal, the number of data to be averaged in the averaging section is preferably variable. Specifically, when the SN is poor, the number of averaging is increased to improve the accuracy of monitoring of the phase noise. On the other hand, when the variation speed of the phase noise is high, the number of averaging is preferably reduced. In addition, since the SN and the phase noise of the signal depend on the transmission distance of the light signal, the number of relays involved in transmission and the like, adjustment is preferably performed accordingly.
In view of the foregoing, various techniques have been disclosed for realizing the averaging section in which the number of data to be averaged is variable.
In the related art described above, however, the delay amount is changed with the change of the number of data to be averaged obtained from the shift register. Specifically, in the abovementioned related art, the data to be referenced is one which is placed at the center of the data to be averaged from the left end of the shift register that corresponds to the input side. Thus, in the related art described above, the delay amount is changed with the change of the number of data to be averaged obtained from the shift register. In other words, when the number of data to be averaged in the averaging section is changed, the delay in the delay addition section needs to be changed. The problems are also found in any of the modulation methods mentioned above.